1. Field of the Invention
The present invention relates to a method for manufacturing a sensor element for a gas sensor, and particularly relates to a method for stabilizing an electrode state.
2. Description of Related Art
Conventionally, various gas sensors have been used for recognizing a concentration of a desired gas component in a measurement gas. For example, as a device for measuring a NOx concentration in a measurement gas such as a combustion gas, known is a NOx sensor having a sensor element which is formed using an oxygen-ion conductive solid electrolyte such as zirconia (ZrO2) (for example, see Japanese Patent Application Laid-Open No. 2006-284223 and Japanese Patent No. 3537983).
In sensor elements of gas sensors including the NOx sensors disclosed in Japanese Patent Application Laid-Open No. 2006-284223 and Japanese Patent No. 3537983, a concentration of an object component in a measurement object gas is obtained by utilizing the fact that in a case where the measurement object gas component is decomposed at a measuring electrode, the amount of oxygen ion occurring at that time is proportional to a current flowing in the measuring electrode and a reference electrode. To be specific, a concentration value of the object component is recognized as follows: a relationship (sensitivity characteristics, a concentration profile) between a concentration value and a current value (output signal value) in each individual sensor element is obtained in advance by using a mixed gas whose object component concentration is already known; and in an actual use, a measured current value is converted into a concentration value based on the sensitivity characteristics.
Therefore, it is ideal that the current value is zero in a case where the object component does not exist in the measurement gas. However, actually, oxygen originally existing in the measurement gas is, though removed prior to the decomposition of the object gas component, left in a small amount and is decomposed, so that a current slightly flows. Accordingly, it is normal that prior to use, a current value (derived from residual oxygen and the like) under a state where the object component does not exist is identified as an offset value, and a value obtained by subtracting the offset value from a current value obtained under a state where the object component exists is used as a current value which is proportional to a gas concentration.
The above-described sensitivity characteristics are determined before each gas sensor is used (for example, before shipment), and normally dealt as fixed characteristics without being changed when the gas sensor is used afterward. This is based on the assumption that actual sensitivity characteristics do not vary during the use of the gas sensor. If the actual sensitivity characteristics change over time, the concentration value which is calculated based on the sensitivity characteristics determined at the time of shipment loses its reliability as the use of the gas sensor continues, and eventually the gas sensor loses a measurement accuracy set in its specification.
To ensure the measurement accuracy, in a manufacturing process of a sensor element, inspection of device characteristics is performed to evaluate electrical characteristics of a sensor element, and if sensitivity characteristics of a sensor element vary or do not satisfy a predetermined specification (management range), it is determined that the sensor element is a defective product.
For improvement of the yield of the product, it is necessary to reduce occurrence of the defective product. The inventors of the present invention have conducted intensive studies, and consequently found out that, for the reason of instability of an oxidation-reduction state in various electrodes provided within the sensor element, that is, the fact that the degree of oxidation at the time of baking an element and the degree of reduction caused by a subsequent aging process performed in a rich atmosphere are not constant, and for the reason that a rich component remaining within the aged sensor element is unexpectedly decomposed, an electrode state in the sensor element after the aging process is instable. And they also have found out the fact that the inspection of device characteristics and the determination of sensitivity characteristics have been conventionally performed under such an instable condition.